Strain Hardening

Strain hardening is also called as work hardening or cold working. It is the strengthening of a metal by plastic deformation. This strengthening occurs due to the dislocation movements within the crystal structure of the material. Low carbon steel which is not amenable to heat treatment is also strain hardened. The materials like pure copper and aluminum only can be strength hardened by strain hardening. Depends on the context, strain hardening can be desirable or undesirable. An example desirable strain hardening is that during metal work process, that intentionally bring the plastic deformation to exact shape change. An example for undesirable strain hardening is during machining, the early passes of a cutter unintentionally strain harden the work piece surface and causing damage to the cutter piece. This process is called cold working process.

The true stress increases continuously in the plastic region i.e. more stress is required to produce additional plastic deformation, when the metal is strained beyond the yield point, and at the same time metal seems to become more stronger and harden, and it is difficult to deform. This shows as the strain increase, the metal will become stronger. It is also referred as strain hardening. The material, which doesn’t show any strain hardening, is classified as perfectly plastic. Such materials would show the constant stress flow irrespective of strain.

Advantages and Disadvantages of Strain Hardening

Advantages

• It requires no heating
• Better surface finish
• Dimensional control is superior
• Better reproducibility and interchangeability
• There are minimum contamination problems

Disadvantages

• It requires the greater forces
• Stronger tooling is required
• It is less ductile
• Always the metal surface should be clean and scale free
• Undesirable lasting stress may be produced.

Strain Hardening in Fatigue

The rate of strain hardening will become almost uniform, if the true stress is plotted against the true strain. Therefore, a metal with high shear modulus will have high strain hardening coefficient (ex molybdenum). The material with small grain size will be more strain hardened than the material with large grain hole.

Strain hardening is closely related to fatigue. Bending the thin steel rod becomes more difficult the further than rod is bent. This is the result of strain hardening. The strain hardening reduces ductility, and increases the chances of brittle failure.   To treat the materials strain hardening is used. By strain hardening the materials will strengthened.

Questions:

• What is strain hardening?
• What are the disadvantages in strain hardening?